M&S says

These firm control knickers flatten tummies and slim hips for a sleeker, slimmer silhouette. Our full shaping brief style gives the most cover, with a high waist, a low cut on the legs and full bottom support. They will discreetly flatten your stomach and slim your hips for a sleeker silhouette. This innovative ultra-light microfiber fabric promises to offer as much control as the traditionally heavy shapewear fabrics, making Secret Slimming™ products as supportive but much comfier.